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Author Topic: creating e.g. vents/extract on angled geometry  (Read 3167 times)

Shane

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creating e.g. vents/extract on angled geometry
« on: November 28, 2013, 06:48:46 am »

Hi all,

Can anyone advise me how to add e.g. an extract fan to angled geometry.

So for example, lets say I make a pitched roof out of two blockage elements, both are angled up to meet at the ridge in the middle.

I now want to add an extract fan somewhere on the angled roof. Does anyone have any idea?

I could make a hole easily enough, but not a fan.....


(This is a simplified version of my problem, I am reading in STL data for a building and a big roof space that encloses it, and none of the roof planes are aligned with x/y/z axes. Thats architects for you!)

many thanks
Shane
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Jim B

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Re: creating e.g. vents/extract on angled geometry
« Reply #1 on: November 28, 2013, 07:24:45 am »

When creating an exhaust or supply surface on a pitched object I generally use the "vent" tool and assign the surface that way.

Otherwise, I create an object, assign my desired surface and snap that to the roof/wall/etc. I then reduce the new object's dimensions until it is close to being flush.

A small warning, assigning an angled exhaust surface can give some odd velocities in addition to adding (literally) weeks to the FDS calculatuions.

I had a problem where I was required to put in an angled exhaust surface who's calculated velocities were ~3 m/s but because the geometry was angled, I got velocities upwards of 30 m/s (I imagine due to clipping issues). FDS also claimed it would take ~700 hours to complete the calculations, instead of merely a day. When I straightened the surface, the results were much more realistic.


Hope that helped,
Jim
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Shane

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Re: creating e.g. vents/extract on angled geometry
« Reply #2 on: November 28, 2013, 09:41:16 am »

Thanks Jim,

as most of these surfaces are say no more than 10deg. from the horizontal, I may as well simplify, and assume they are flat given what you say about run times.

Sorry if this is obvious but: if I define an obstruction to act as a roof, and I want to put an extract fan on it, do I put the vent surface "underneath" the obstruction  (and so FDS assumes the air flow is lost from the domain at that point) or do I put the vent surface on top (but then FDS would have to "punch through" the roof to get the vent to the space below?


(I've attached a vent to a roof but it does not seem to be removing air)

Thanks
Shane
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Jim B

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Re: creating e.g. vents/extract on angled geometry
« Reply #3 on: December 02, 2013, 02:15:13 am »

Shane,

Sorry for the late reply. You want to put it underneath, otherwise it will be removing air that is not "within the model".


Let me know if that works,
Jim
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